There is a theorem in simplicial homology theory which states that
any continuous tangent field on a 2-sphere is null at least in a point.
Mathematically literate hackers tend to associate the term
‘hairy’ with the informal version of this theorem; “You
can't comb a hairy ball smooth.” (Previous versions of this entry
associating the above informal statement with the Brouwer fixed-point
theorem were incorrect.)

The adjective ‘long-haired’ is well-attested to have been
in slang use among scientists and engineers during the early 1950s; it was
equivalent to modern hairy senses 1
and 2, and was very likely ancestral to the hackish use. In fact the noun
‘long-hair’ was at the time used to describe a person
satisfying sense 3. Both senses probably passed out of use when long hair
was adopted as a signature trait by the 1960s counterculture, leaving
hackish hairy as a sort of stunted
mutant relic.

In British mainstream use, “hairy” means
“dangerous”, and consequently, in British programming terms,
“hairy” may be used to denote complicated and/or
incomprehensible code, but only if that complexity or incomprehesiveness is
also considered dangerous.